In this fiscal year, we continued to explore the functional role of myositis autoantigens in muscle differentiation and repair. As in other systemic autoimmune diseases, patients with myositis often have autoantibodies targeting self-proteins. We previously demonstrated that the dermatomyositis (DM) autoantibody CHD4 is up-regulated in regenerating muscle and that reducing levels of this protein in cultured muscle cells accelerates their differentiation. Recently, we showed that another DM autoantigen, TRIM33, is similarly up-regulated during muscle repair and seems to play a role during muscle differentiation in cell culture. During this year, we studied muscle differentiation and regeneration using mice that have no CHD4 or TRIM33 expressed in muscle cells. These experiments show that knockout of CHD4 is embryonic lethal and that both of these genes play a role in muscle regeneration following muscle cell injury. Using these mice (as well as muscle cells cultured from them), we are now determining the molecular mechanisms by which CHD4 and TRIM33 modulate muscle differentiation and repair. In addition to the basic science project described above, we also completed several clinical projects involving myositis patients who are part of the Childhood Myositis Heterogeneity Collaborative Study or the Johns Hopkins Myositis Center Longitudinal Cohort. These projects included: (a) Analyzing a longitudinal cohort of pediatric myositis patients with anti-HMG-CoA reductase (HMGCR) antibodies. This published study showed that anti-HMGCR autoantibodies are not just found in adults, as previously described by our group, but also in children with myositis. (b) Studying pathologic mechanisms in sporadic inclusion body myositis. Using muscle biopsy samples from patients with various forms of myositis, we published a paper demonstrating that calcium dysregulation, calpain deficiency, and endoplasmic reticulum stress play a role in damaging muscle in patients with sporadic inclusion body myositis. (c) Defining unique phenotypes associated with distinct myositis autoantibodies. Using a longitudinal cohort of adult myositis patients, we published an paper showing that patients with anti-NXP-2 autoantibodies have a unique phenotype including edema, dysphagia, and distal weakness. Using the same cohort, we published another paper showing, among other things, that older anti-HMGCR positive patients have a better outcome than younger patients with the same autoantibody. In collaboration with Dr. Lisa Rider, Dr. Fred Miller, and their colleagues at NIEHS, we have sought to determine whether pediatric myositis patients ever have anti-NT5C1a autoantibodies. We screened a large cohort of pediatric myositis cases and found that 27% were anti-NT5C1a positive. Interestingly, anti-NT5C1a positive patients have more severe disease manifestations than those without this autoantibody. This work has been submitted for publication.